A standard four-high roll stand comprises a frame, an upper backup roll and a lower backup roll rotatable in the frame about respective vertically spaced axes, and an upper working roll and a lower working roll flanked by and operatively engaging the respective backup rolls. Actuators urge the backup rolls toward each other and respective upper and lower drives are connected to the respective backup rolls to rotate same and thereby pull a workpiece engaged between the working rolls through the roll stand. An inner backup roll may be provided in a six-high arrangement between each of the outer backup rolls and the respective working roll. The rolls may be cylindrical or even bottle-shaped (See German patent No. 3,038,865 and U.S. patent application No. 311,449 (now U.S. Pat. No. 4,440,012) filed Oct. 14, 1981), and can be relatively axially shiftable.
For the size reduction of the workpiece the outer backup rolls are urged toward each other with a high force and both of the drives apply considerable torque to these backup rolls. The force is transmitted frictionally to the working rolls which in turn engage and pull the workpiece. The drives are rated such that they operate at high efficiency in this high-force mode.
For various roll-type finishing operations the squeeze force applied to the workpiece is substantially less, as no appreciable thickness reduction is wanted. In such operation the frictional coupling between the backup rolls and the respective working rolls, as well as between the workpiece and the working rolls, is substantially reduced. The drives must therefore be operated to deliver a fairly low torque to the backup rolls, in a range in which the respective motors normally are very inefficient. Thus when such a roll stand is applied to a light-force application it operates inefficiently.
In addition in the low-force mode, which can be only one-tenth the force of the high-force mode, there is a tendency of the system to vibrate. This has been traced in part to small variations in the drive speeds of the two backup rolls, variations which are impossible to eliminate completely. Such problems are aggravated even more in arrangements wherein the plane of the working-roll axes is offset parallel to the workpiece-travel direction from that of the backup rolls, as suggested in application Ser. No. 710,837, filed Mar. 12, 1985.